A silent killer called "shallow water blackout" can affect both kids and adults who swim and it has claimed the lives of even the most accomplished swimmers.

It happens when someone is underwater for too long.

Most of the time, your body will tell you when you need to come up for air - but that's not always the case.

Action News spoke with a local coach and swimmer about the danger.

18-year-old Colin McHugh swims competitively but knows his limits when it comes to being underwater. In fact, he's felt when he pushed it too far.

"Ten meters left in the pool, I started getting really dizzy like I was going to go somewhere and I popped up immediately because I knew I needed to breath," he recalled.

But Matthew Sprang, coach and owner of the Greater Philadelphia Aquatic Club, says the signal that McHugh needed to breathe doesn't always come.

If someone is hyper-ventilating between sets, their oxygen is already low and their carbon dioxide also drops.

"The fact that you have low levels of oxygen is going to cause you to faint. The fact that you have low levels of CO2 is going to cause your brain not to realize you need to come up and take a breath," said Sprang.

An underwater camera shows just how quickly a swimmer can faint. If someone doesn't catch the swimmer fast, he could drown.

It's a danger top USA swimming coaches and even Olympian Michael Phelps have been working to prevent through a public service announcement.

The PSA is not just a message for competitive swimmers, but also everyday kids swimming in neighborhood pools.

To prevent it: never swim alone. Second, don't ignore the urge to breathe and don't attempt long or repetitive underwater swims. And finally, never compete in breath-holding games

"Games like sharks and minnows, games like how long can you hold your breath, those can be very dangerous," said Sprang.

And if you're practicing like Colin McHugh, be sure to catch your breath between sets.